I. Field of the Invention
This invention relates to a method and apparatus used for the direct coating of strip articles, particularly metal sheet, with paints, plastics or other coating materials, provide surface protection and/or to improve the appearance of the article.
II. Description of the Prior Art
Metal sheet material, and particularly thin aluminum strip used for building siding, beverage can stock and other purposes, is frequently coated with materials such as paints or plastics to provide surface protection and/or decorative finishes. The coatings are typically applied by dissolving or suspending polymers and other components in organic solvents, applying the resulting mixtures by roller coater to the strip article, and baking the resulting product to remove the solvents and to cross-link the polymers.
An alternate means of coating sheet articles is to employ a coating die in the form of a coating head having an elongated open sided coating slot arranged transversely to the strip through which the coating material is extruded directly onto the strip surface as in is moved past the coating head. The thickness of the coating produced in this way can be controlled by using a spacer of some kind (e.g. a roller "doctor blade") that separates the coating head from the surface of the strip article and rides along the surface of the strip as the coating is applied. Such a spacer maintains the height of the coating gap (the gap between the coating head and the surface of the strip article) at a constant, preset value regardless of any lack of uniformity of the strip or strip feed mechanism.
Another way of maintaining a constant coating gap support the ends of the coating head that extend transversely beyond the edges of the strip article by means of a supporting structure attached to a frame carrying the strip article feed mechanism. The supporting structure maintains the desired coating gap, but nothing contacts the surface of the strip article, thus reducing the possibility of marking of or damage to the surface to be coated.
Yet another way of maintaining a desired coating gap is to use a coating head that "floats" on the layer of coating material as it is applied to the surface of the strip article. An apparatus of this kind suitable for single-sided coating of sheet material without reliance on mechanical spacers is disclosed in U.S. Pat. No. 4,675,230 of Jun. 23, 1987, assigned to the same assignee as the present application (the disclosure of this patent is incorporated herein by reference). Moreover, a related apparatus and method of two-sided coating of sheet material is disclosed in pending patent application Ser. No. 08/068,990, filed May 27, 1993 and assigned to the same assignee as the present application (the disclosure of which application is also incorporated herein by reference). The types of apparatus disclosed in this patent and patent application rely on the hydrodynamics of the coating material as it is applied to the metal strip for control of the film thickness and can readily compensate for variations in the gauge of the strip and eccentricity of the support roll. This is achieved by using a coating head having an extended surface on the downstream side of the coating slot forming an angle (normally in the range of 0.1.degree. to 5.degree., or more preferably 0.5.degree. to 1.degree.) with the moving strip creating a coating gap converging in the direction of the strip advance. The extended surface directly contacts the coating material as it is applied to the strip generating hydrodynamic forces that cause the head to "float" on the layer of coating as it is applied to the strip. A load is applied to the coating head to counter balance the hydrodynamic forces to maintain a constant coating gap. Direct contact between the strip and the coating head is thus avoided, and this in turn avoids damage to or defacement the metal or pre-coated metal surface to which the coating is being applied.
While direct coating devices of the types mentioned above are effective for strip coating, they suffer from the disadvantage that inadvertent uncovering of the ends of the coating slot by errors in strip tracking or by variations in strip width can result in extensive leakage of the coating material from the slot, which normally requires stoppage of the coating line to correct the condition and to clean up the spilled coating material. To reduce the likelihood of this happening, it has been necessary to leave uncoated bands, often several millimeters wide, at the sides of the strip article to provide safety zones within which the positions of the side edges of the strip may vary without uncovering parts of the slot. For some articles, such as aluminum siding, this has been unimportant because the edges of the sheet are concealed in the final product. For other products, such as beverage can end stock, the edges are frequently trimmed after coating and there is anyway a 1 to 2 mm wide edge band left on the "skeleton" after the lids have been punched out. However, there are products that make use of the full width of the coated strip article, and even with can end stock it is desirable to coat the strip as close as possible to the strip edges for aesthetic if not for functional reasons.
There is therefore a need for a method and apparatus for coating strip articles in which the coating can be brought closer to the strip edges without increasing the risk of substantial spillage of coating materials.